Many types of mass analyser have been developed to date and they can be divided into two categories depending on the way they detect an ion signal. One category of mass analyser, referred to as a destructive detection mass analyser employs a Faraday cup or secondary electron multiplier and has been widely used in quadrupole or quadrupole ion trap mass spectrometers, in sector magnetic deflection mass spectrometers and in time-of-flight mass spectrometers. In these mass spectrometers, following the selection/separation process in the analyser, ions splash onto the electrode of the detector and disappear.
Another category of mass analyser, referred to as a non-destructive detection mass analyser, normally detects an induced charge in a pick-up electrode which is called the image charge detector. The induced image charge varies when the measured ion is passing by the detector surface resulting in an image current in a circuit connected to the measuring device. Such methods have been used in FTICR, first disclosed in M. B. Comisarow and A. G. Marshall, Chem. Phys. Lett. 25, 282 (1974), and were introduced later into the so-called Orbitrap by Alexander Makarov, disclosed in Anal. Chem., 2000, 72 (6), pp 1156-1162. In these devices the ions that contribute to image current being detected are not lost during the detection procedure so they can be measured many times in the analyser, giving rise to a higher mass resolution and better mass accuracy.
An electrostatic ion trap is more attractive because it avoids use of a high strength and high stability superconducting magnet. The Orbitrap is one example of an electrostatic ion trap where ions can keep oscillating in the axial direction while, at the same time, rotating around a central spindle-shaped electrode. To keep the axial oscillations harmonic, the central and outer electrodes of the Orbitrap need to be very accurately machined so as to achieve a so-called hyper-logarithmic potential inside the trap volume. In U.S. Pat. No. 7,767,960B2, Makarov disclosed some alternative forms to create the hyper-logarithmic potential where an array of cylindrical electrodes are used to mimic a single, complex-shaped electrode, so that any machining error might be compensated electrically using a “tuning procedure”.
It is not necessary for the electrostatic trap to have a field structure that allows ions to perform harmonic motion in any one direction, such as in the Orbitrap. An electrostatic ion beam trap (EIBT), which uses isochronous mirrors, can also be used for mass analysis with image charge detection. Strehle Frank in DE4408489A disclosed a coaxial, double mirror, multi-turn trapping device that can be used for mass analysis by Fourier transformation of the image current detected by a pair of pick up electrodes. H. Benner in U.S. Pat. No. 5,880,466A disclosed an analyser having a single, cylindrical pick-up electrode for highly charged protein analysis. Zajfman WO02103747 (A1) also disclosed a modified device of the same form for general mass analysis.
One of the big issues in image current detection using an electrostatic trap is the dynamic range of the ion signal. The minimum detectable mass peak relates to the induced image charge derived from the number of ions, having the same mass to charge ratio, that is comparable to the noise of the detection circuit, and so far this is down to about 10 ions in the Orbitrap. The upper limit of the mass peak, on the other hand, is defined by the space charge derived from the number of ions in the mass peak that affects the measurement of a neighboring peak. This is normally about 10,000 for high mass resolution measurement.
To reduce the lower detection limit, use of multiple pick-up cylinders and a new conversion algorithm making use of multiple harmonic components in the image current signal have been proposed by Ding in US patent application 200810207492.6. While these developments have the potential to improve the resolution and the lower detection limit, the use of a narrow beam type of reflector and cylindrical pick-up electrodes restricts the maximum number of ions that can oscillate in the device without suffering space charge effects.
In US patent application US 2010/0044558 A1 Sudakov disclosed a multiple reflection time-of-flight device constructed by using a pair of planar electrode arrays. Ions are reflected in a flight direction (x) by two mirrors formed by parallel electrode strips in the planar arrays, and in a drift direction (z) by one mirror formed by another set of electrode strips on the same planar arrays. Isochronous motion of ions of the same mass-to-charge ratio is achieved in the (x-axis) flight direction within each cycle, but only for one reflection in the (z-axis) drift direction. As the ions are not tightly focused in the drift direction, Coulomb interaction between the ions is relatively small, thus giving rise to a higher space charge tolerance.
It would be desirable to have a multiple reflection type of electrostatic ion trap with image current detection for use as a mass analyser, which combines the merits of easy construction, ease of ion injection, high space charge capacity, high sensitivity (lower limit of detection) as well as high mass resolution.